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Re: Counting number of numbers in a large list between two valus

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114524] Re: Counting number of numbers in a large list between two valus
  • From: Ray Koopman <koopman at sfu.ca>
  • Date: Mon, 6 Dec 2010 06:15:05 -0500 (EST)
  • References: <idhjda$9f6$1@smc.vnet.net>

On Dec 5, 6:56 pm, Lyle <lgor... at gmail.com> wrote:
> Dear Listers,
>
> I have a large (5-20million) one dimensional list of real numbers and
> I want to count the number of entries in the list that lie between 2
> specific values (x1, x2). I need to run the function for a number of
> different ranges.
>
> ie. number of list entries (l), where x1 <= l <= x2
>
> I've tried:
>
> tallydata[{x1_, x2_}] := Count[data, x_ /; x1 <= x <= x2]
>
> that takes about 3-4 seconds
>
> and
>
> tallydata[{x1_, x2_}] := Length[Select[data, x1 <= # <= x2 &]]
>
> which takes a little bit longer.
>
> The best I've managed is (this last one might be off by 1 or 2 but
> this doesn't really matter to me):
>
> sorteddata = Sort[data];
> nf = Nearest[sorteddata];
> tallyrange[{x1_, x2_}] :=
>  First[Position[sorteddata, First[nf[x2]]]] -
>   First[Position[sorteddata, First[nf[x1]]]]
>
> which takes between 1 and 2 seconds but I was hoping there might be a
> faster way to do this?
>
> Any help would be great!
>
> Thanks,
> Lyle Gordon
>
> Northwestern University

data = RandomReal[1., 2*^6]; min = .2; max = .3;

Timing@Length@Select[data, min <= # <= max &]
{8.76, 200028}

Timing@Count[data, x_ /; min <= x <= max]
{7.99, 200028}

Timing@Tr@UnitStep[(data - min)*(max - data)]
{0.61, 200028}


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